Can you download a version of blender and still have access to the previous version
As far as I know, if you create a project in Blender 2.8 you can't open it with the 2.79 version. But you can open 2.79 projects with the 2.8 beta version, but if you save it using the 2.8 you won't be able to open it again with the 2.79.

When you download the 2.8 beta version, it isn't a setup (because is beta) so you have a portable file. That means that the desktop shortcut will be for the 2.79 and you have to create a new one for the 2.8 looking for the exe, so you could have both versions (actually I have the 2.79 shortcut next to the 2.8, both avaliable).